Columns. The first row names the columns. phone is the only one required; first_name, last_name, email, company, groups and carrier are optional and may come in any order. Other columns are ignored.
Numbers. US numbers in any common form: +13105550123, (310) 555-0123 or 3105550123. A row whose number cannot be read is skipped and listed with its line number.
Groups. Several groups go in one cell separated by semicolons: Patients;VIP. Groups that do not exist yet are created.
Carrier. If your previous system already knew the carrier, put it in the carrier column: AT&T, T-Mobile or Verizon, including the brands that ride on them (Cricket and Consumer Cellular count as AT&T; Metro, Mint, Boost and US Cellular as T-Mobile; Visible, Xfinity Mobile and Spectrum as Verizon). The number is then treated as a mobile that can receive SMS, Run lookup skips it, and no lookup tokens are spent on it. A spelling we do not recognise is ignored and listed; the contact is still imported.
Existing contacts. A number already in the book is updated, not duplicated: only its blank fields are filled in, and groups are added, never removed.
Opted out. Numbers that opted out import like any other and stay excluded from every send.